In recent years there has been an explosion of work demonstrating the formation of oil in water or water in oil droplets within microfluidic devices. The interest was initiated by pioneering work of the groups of Quake, (T Thorsen, R W Roberts, F H Arnold, and S R Quake, PRL 86, 4163 (2001)), Weitz (A S Utada, L-Y Chu, A Fernadez-Nieves, D R Link, C Holtze, and D A Weitz, MRS Bulletin 32, 702 (2007)) and Stone (S L Anna, N Bontoux, and H A Stone, Appl. Phys. Lett. 82, 364 (2003)), these papers both elucidating the behaviour of concentric multiphase flows and demonstrating exquisite control over synthesis of multiphase droplet systems. In all cases, the fundamental microfluidic component is a flow focusing arrangement that brings together two immiscible phases. Cascading such components has enabled water-in-oil-in-water-in-oil etc. systems to be created. Further, such microfluidic devices may be used as a general fabrication route to precisely control monodisperse materials, although such elemental devices would need to be fabricated massively in parallel in order that useful quantities of material may be made. Planar flow focusing devices have the utility of easy fabrication through the now well known PDMS fabrication process. Since PDMS is an intrinsically hydrophobic material it has been readily utilised to make water-in-oil systems that have been the particular focus for biological investigation where each droplet can be used as a reactor, for example for PCR reactions.
The particular interest in these microfluidic flow focusing systems stems from their ability to form precise monodisperse droplets, usually at rates up to a few kHz. Several papers have demonstrated that the formation of monodisperse droplets is the result of a flow instability associated with the two phase flow within a nozzle. Guillot et al (P Guillot, A Colin, A S Utada, and A Ajdari, PRL 99, 104502 (2007)) have shown that the flow instabilities associated with multiphase flow in such a flow focusing device can be described as either absolutely unstable, i.e. a dripping mode, or convectively unstable, i.e. a jetting mode. The jetting mode is a generalisation of the well known Rayleigh-Plateau instability of a free jet. A jet of one liquid within another will disintegrate into a series of droplets with a well defined average wavelength and therefore size irrespective of the flow rate. However in contrast to the flow focusing dripping mode the droplets will in general be polydisperse. In order to form monodisperse drops either the dripping or the geometry controlled drop formation mode is required. Utada (A S Utada, A Fernandez-Nieves, H A Stone, and D A Weitz, PRL 99, 094502 (2007)) has demonstrated that these modes are constrained to finite Capilliary and Weber number (Ca, We), that is the region where the growth of a perturbation propagates both upstream and downstream and is therefore absolutely unstable.
In order to take the exquisite control of droplet formation and synthesis afforded by microfluidic systems to a practical drop fabrication methodology, the ability to generate monodisperse droplets at significantly higher frequency is required. Further such methods then also become potentially useful as droplet generators for continuous inkjet.
WO2009/004314 and WO2009/004312 are examples of droplet formation in microfluidic devices.
Flow focusing devices are now well known in the art, for example see US2005/0172476. In these devices a first fluid phase that will become droplets is introduced via a middle channel and a second fluid phase that will become the surrounding carrier phase is typically introduced via at least two separated and symmetrically placed channels either side of the middle channel (systems with a single carrier phase channel also being known). Provided the walls of the channels supplying the carrier phase and the outlet channel are preferentially wetted by the carrier phase it will completely surround the first fluid phase which then breaks into droplets as the fluid flow is forced through a nozzle formed within the device.
In the prior art a common occurrence of obstructions in the context of a microfluidic device is by way of an array of pillars, in some instances activated or with a surface coating, that is used as an in-line filter or collection device (see for example US2008/0044884). These pillars are not intended to cause significant turbulence to the bulk flow. US2005/0161326 discloses in one embodiment an array of pillars in the flow channel slightly downstream of the intersection of the flow of two separate fluids. The pillars are deliberately added to cause non-laminar flow to aid the mixing of the two fluids to promote chemical reaction between the components, the two fluids being therefore miscible. WO2006/022487 also discloses an array of pillars in a flow channel but as a means of accelerating flow in the channel through an increase of the capillary force on the fluid. This usage is to quantitatively regulate the flow of a single fluid in a microfluidic device used for analytic or diagnostic purposes.
There is also earlier described a form of homogeniser which used ultrasonic vibration to break up into small droplets a jet of the liquid. The “Pohlmann Whistle” (see “Ultrasonics”, p 147 to 149, by A. P. Cracknell, Wykeham publication 1980) is a jet-edge system in which the two liquids to be homogenised are forced through a nozzle to form a rectangular shaped jet which impinges upon a bevelled edge of a steel blade positioned downstream from the nozzle, which blade is typically cantilever mounted and which vibrates at its natural frequency being that required to break up the jet stream. High fluid velocities are required to induce this homogenisation and droplets formed are polydisperse.
The prior methods of size-controlled droplet generation of a fluid are limited to the low frequency dripping method referred to above (that is their frequency is limited by the necessity to keep the system in an absolutely unstable, i.e. dripping, regime) and active intervention method such as that in a continuous inkjet printhead using piezo crystals to generate droplets from a fluid.